The present invention relates to a light-sensitive composition used for, in particular, preparing a presensitized plate for use in making a lithographic printing plate and more specifically to a light-sensitive composition which is excellent in developability with an aqueous alkaline developer, ensures the formation of a lithographic printing plate having good printing durability and is improved in storage stability.
The majority of the compounds used as light-sensitive substances for printing materials to which light-sensitivity is preliminarily imparted are diazonium compounds and most commonly used are diazo resins represented by a condensate of p-diazodiphenylamine with formaldehyde. When such a diazo resin is applied onto the surface of an appropriate substrate such as paper, a plastic film or a metal plate and then exposed to actinic light rays through a negative transparency, the diazo resin present in the exposed area of the light-sensitive material is decomposed into an insoluble substance. Therefore, the unexposed area thereof can be removed by dissolving in an appropriate developer to thus reveal the surface of the substrate. For this reason, if a substrate whose surface has previously been hydrophilized is used, the hydrophilized surface thereof behind the unexposed area is revealed through development. The revealed surface thus receives water but repels an ink during printing operation using, for instance, an offset printing press. On the other hand, the exposed area (or the decomposed diazo resin) of the light-sensitive material has lipophilicity and repels water, but receives an ink. In other words, the printing material of this kind provides a so-called negative-positive type printing plate.
The light-sensitive composition of this type has low storage stability and accordingly requires the use of a stabilizer. There have been proposed various such stabilizers, for instance, phosphorous acid as disclosed in Japanese Unexamined Patent Publication (hereinafter referred to as "J. P. KOKAI") No. Sho 52-151023, halogen atom-containing organic phosphoric acid ester compounds as disclosed in J. P. KOKAI No. Sho 50-36207, oxalic acid as disclosed in U.K. Patent No. 548,344, high molecular weight organic acids as disclosed in J. P. KOKAI No. Sho 56-107238 and malic acid as disclosed in J. P. KOKAI No. Sho 60-262154. These stabilizers are only slightly effective and still insufficient.
The compositions comprising diazo resins used for forming the light-sensitive layer of presensitized plates for use in making lithographic printing plates (hereinafter referred to as "PS plates") can be divided into those comprising only diazo resins and free of binders as disclosed in U.S. Pat. No. 2,714,066 and those comprising binders and diazo resins in combination as disclosed in, for instance, U.S. Pat. No. 4,123,276. However, most of the recent PS plates comprising diazonium compounds simultaneously contain polymers serving as binders for the purpose of imparting high printing durability to the resulting printing plates.
As such light-sensitive layers, there have been known so-called alkali-developable ones whose unexposed area is removed (developed) with an aqueous alkaline developer such as those disclosed in Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") No. Sho 52-7362 (=U.S. Pat. No. 4,275,138) and so-called solvent-developable ones whose exposed area is removed with a developer comprising an organic solvent. Among these, the former has attracted special interest from the viewpoint of industrial safety and health.
However, the alkali-developable light-sensitive layers in general have insufficient printing durability and accordingly, there has been desired for the development of light-sensitive layers which simultaneously satisfy the requirements of high alkali-developability and printing durability.